Zirconium has a very low neutron capture cross section and, for this reason, is used in nuclear reactors. the ore zircon, however, always contains hafnium, in a proportion of approximately 1 to 3% by weight. In contrast to zirconium, hafnium strongly absorbs neutrons. The use of zirconium in the nuclear field, therefore, requires the preliminary removal of hafnium, a content of less than 100 ppm often being recommended.
Zirconium has other applications in which its purification is desirable. For example, the removal of hafnium is also sought in stainless superalloys.
Hafnium and zirconium have very similar properties and their separation. is thereby rendered extremely difficult. To do this, a carbochlorination of the ore is performed, which produces the tetrachlorides ZrCl4 and HfCl4, and then a separation, either by extractive distillation of the two tetrachlorides or by liquid—liquid extraction after dissolving the chlorides, is carried cut. In the latter case, this results in the formation of ZrO2 and HfO2, which then requires a further carbochlorination of the zirconium before moving onto the phase of recovery of the metal Zr. The first process, which is more efficient, consists of a distillation of molten salts which makes it possible to retain the tetrachloride form, thus dispensing with the second carbochlorination. The temperature is generally comprised between 250 and 550° C. See, for example, French Published Patent Application No. FR 2 250 707 and U.S. Pat. No. 4,021,531.